The present invention relates to an apparatus and process for removing dissolved, suspended and particulate matter from liquid containing same, and especially concerns the removal of color-,taste-, odor-, and turbidity-producing impurities from water supplies and waste streams. More particularly, this invention relates to the filtration, clarification and purification of water and to a novel apparatus for achieving such treatment.
Conventional water treatment facilities are effective for removing from water a wide range of impurities including particulate matter, such as mineral fibers, clays and microbial substances, as well as suspended and dissolved matter, e.g. chlorinated organics and various other color-,order-,taste-, and turbidity-producing impurities.
Many municipal and industrial water treatment plants employ systems in which raw water is pretreated, typically by coagulation/flocculating and sedimentation, after which it is filtered and disinfected. Municipal water may be treated further by using activated carbon, oxidation and aeration, when necessary for taste, order or bacteria control. Demineralization and ultrafiltration are also employed for certain industrial uses where complete removal of dissolved matter is required.
Coagulation is ordinarily carried out in a rapid mixing tank by adding to the raw water a chemical agent which causes agglomeration of the suspended matter into larger particles which settle to the bottom of a containment vessel. The agglomerated particles, or floc, are sometimes subjected to gentle agitation in order to form floc bundles large enough to settle rapidly. A separate flocculation tank is often provided for this purpose.
Sedimentation is the process by which floc is separated from water by precipitation and deposition and depends on the effect of gravity on particles suspended in a liquid of lesser density. Sedimentation without pre-treatment is rarely adequate for clarification of turbid water, for example, as it does very little for removal of fine particulate substances such as clay, bacteria, and the like. Also sedimentation results in sludge formation, requiring that sludge disposal means be provided.
After the raw water has undergone appropriate pre-treatment, it is filtered for the removal of remaining suspended particles by passage through a porous medium. Most conventional water treatment systems of this type employ granular media filtration. For example, rapid sand filters and mixed media filters (e.g. sand and anthracite coal) have been applied on a municipal scale for many years. The filtrate or effluent is then treated with a disinfectant, or sterilant, typically chlorine or ozone. Further processing for taste, odor and bacteria control typically involves additional specialized equipment, including activated carbon columns, aerators and treating chemicals, such as chlorine. The same is true in the case of industrial water that requires demineralization or ultrafiltration.
A significant drawback of water treatment systems of the type just described is the substantial cost involved in the installation, operation and maintenance thereof. Moreover, water treatment systems involving granular media filtration are often plagued by short filtration cycles resulting from clogging of the media by appreciable amounts of suspended matter not removed during pre-treatment, and, in the case of mixed media filtration, by anthracite coal fines which accumulate on the upper surface of the media bed after repeated back washing and tend to inhibit effective penetration of impurity particles into the media. Other operational problems that are often experienced in granular media filtration are low filter capacity and channelling.
Although there has been considerable effort toward improving these prior art water treatment systems, the improvements have related generally to enhancing floc formation and sedimentation, reducing the load on the filters, and using larger grain sand and higher filtration rates as a means of reducing cost.
Filter-aid filtration, long used in numerous industries for liquid clarification, has been employed in municipal and industrial water treatment for several decades as an alternative to granular media filtration. Filter-aid filtration offers a distinct economic advantage over granular media filtration in that the need for large and expensive vessels for floc formation and sedimentation is virtually eliminated, since water is seldom, if ever, pre-conditioned for filter aid filtration.
Filter-aid filtration systems commonly consist of an industrial-type filter with a filter membrane arranged usually in a series of leaflets. The membrane is pre-coated on the upstream side with a thin layer of filter aid of a pre-determined particle size, which forms the pre-coat filter cake. The materials most often used as filter aids are diatomaceous earth, perlite, finely divided carbon, and fibrous materials, such as cellulose and various synthetic fibers. As water flows through the cake, the solid impurities present in the water are caught in small pores of the filter cake. Additional filter aid, referred to as "body feed", is normally admixed with the stream of raw water, and the suspended impurities together with the added filter aid are retained on the filter membrane. The layer of sediment and filter aid is subsequently removed by backwashing, which must be rather thorough so that the new pre-coat will readily form on the membrane.
Although the performance of filter aid filtration systems has been satisfactory in many respects, these systems are not without certain deficiencies. For example, water supplies and waste water streams frequently contain substances, such as pathogenic microorganisms, that are too finely divided to be removed by commercially available grades of filter aid materials. While it is possible that a filter aid could be produced which is capable of effectively removing such substances from the filtrate, the liquid flow resistance of such a filter aid material would be so high as to make its use impractical.
The use of resin-bonded fiber filters for liquid filtration is also well known. It has been proposed to improve such filters by the addition of finely divided adsorbing agents, such as activated carbon, to remove from liquids treated therewith certain dissolved substances, e.g. taste-,odor-, and color-producing organic substances, chlorine, pesticides and other toxic organics. Such proposals have not been particularly successful, however, since the resin binder is required in such an amount as to diminish considerably the effective surface area of the particulate adsorbent. Consequently, the adsorption capacity of the filter is substantially reduced, necessitating frequent replacement of the filters. Moreover, resin-bonded fiber filters containing particulate adsorbing agents are often characterized by extremely low fluid permeability, and a high pressure drop, making them inappropriate for many liquid filtration applications, especially in the treatment of waste water streams of high solids content.
While recent developments in the art have somewhat reduced the cost of installation and equipment maintenance, the overall operating expense of present day water treatment facilities remains relatively high. In short, the cost of water treatment, whether by granular media filtration, filter-aid filtration or resin-bonded fiber filters, makes it uneconomic for many smaller size municipalities and industries which might advantageously employ a water treatment plant. Hence, the development of an effective and efficient process for producing substantially pure, clear water for domestic and industrial consumption at a reasonable cost continues to be a highly desired objective.